Cordless telephone set with a battery and a battery charger and having a power supply line setting unit for establishing a power supply line for power supply from the battery or the battery charger

ABSTRACT

A cordless telephone set includes a battery, and a communications part for communicating with another cordless telephone set via a transmission medium including radio channels, the communications part comprising a part which detects a call received via the transmission medium and indicates receipt of the call. A D.C. signal detection unit detects a D.C. signal supplied from a battery charger when a battery is coupled to the battery charger and charged thereby. A power supply line setting unit, which is coupled to the part of the communications part and the D.C. signal detection unit, establishes a power supply line for supplying electricity generated by the battery charger to the part of the communications part when the D.C. signal detection unit detects the D.C. signal.

This is a continuation of application Ser. No. 08/332,458, filed Oct. 311994, now U.S. Pat. No. 5,481,593 which, in turn, is a continuationapplication of Ser. No. 07/912,315, filed Jul. 13, 1992, and nowabandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to cordless telephone sets, andmore particularly to a cordless telephone set having a function ofdetecting a call while a built-in battery built is being charged.

2. Description of Related Art

FIG. 1A shows a related cordless telephone set 10. The cordlesstelephone set 10 is configured as follows. A radio interface unit 1communicates, via a radio channel, with a host device (FIG. 1B)connected to a telephone line TL coupled to an exchange EX. A portabletelephone system is made up of the portable telephone set 10 and thehost device. A speech receiver 2 receives a speech signal transmitted byanother subscriber and received via the host device, the radio channeland the radio interface unit 1. An amplifier 3 amplifies the speechsignal received by the speech receiver 2. An amplified speech signaloutput by the amplifier 3 drives a speaker 4, which converts theamplified speech signal into an acoustic speech signal. A microphone 5converts an acoustic speech signal into a corresponding electric speechsignal. An amplifier 6 amplifies the electric speech signal from themicrophone 6, and outputs an amplified speech signal to a speechtransmitter 7. The speech transmitter 7 generates a radio wave from theamplified speech signal. The radio wave is transmitted via the radiointerface unit 1, a radio channel and the host device. A signal receiver8 receives call control signals output by the above exchange andreceived via the telephone line, the radio channel, the host device andthe radio interface unit 1 and transmits these control signals to aprocessor 11. Examples of these call control signals are a callingsignal, a disconnect signal, a response signal, and a clear signal. Asignal transmitter 9 receives call control signals from the processor 11and transmits these call control signals to the exchange via the radiointerface unit, the radio channel, the host device and the telephoneline. Examples of the call control signals are a call signal, a selectsignal of a caller, a disconnect signal, a response signal, and a clearsignal. The processor 11 controls the entire operation of the cordlesstelephone set 10. More specifically, the processor 11 rings a sounder 14in response to receipt of the calling signal from the signal receiver 8,and outputs the select signal based on a dialed number input by means ofa ten-key keypad 12 to the signal transmitter 9. Further, the processor11 detects the ON/OFF states of a hook switch 13, and outputs thecalling signal, the response signal, the disconnect signal, the clearsignal or the like to the signal transmitter 9.

Electricity necessary to operate the above-mentioned structural elementsis generated by a battery 15, and is supplied thereto via a switch 19and a power supply line 20 or 21. The power supply line 20 carrieselectricity necessary to ring a sounder 14 and thereby notify thesubscriber of receipt of a call to the radio interface unit 1, thesignal receiver 8 and the processor 11. The power supply line 21 carrieselectricity necessary to operate all of the other structural elements.

The switch 19 selects one mode from among three modes, that is, a pausemode P, a standby mode S, and an active mode A. In the pause mode P, anoutput terminal of the battery 15 is completely disconnected from allthe structural elements of the cordless telephone set 10. In the standbymode S, the battery 15 is connected to only the power supply line 20 viathe switch 19. In the active mode A, the battery 15 is connected to thepower supply line 21, and the power supply line 20 via a diode 22.

When the active mode A is selected by the switch 19, electricitygenerated by the battery 15 is supplied to all of the structuralelements, and hence the cordless telephone set 10 is maintained in anactive state in which it can receive a call and transmit a call. It willbe noted that a large amount of energy is consumed in the cordlesstelephone set 10 in the active mode A. When the standby mode S isselected by the switch, electricity is supplied to only the radiointerface unit 1, the signal receiver 8 and the processor 11. Hence, inthe standby mode S, an amount of energy is consumed smaller than thatconsumed in the active mode A. When the pause mode P is selected by theswitch 19, electricity is not supplied to the structural elements atall. Hence, energy stored in the battery 15 is consumed only slightly.In the pause mode P, it is impossible to notify the subscriber ofreceipt of a call.

With the above in mind, it is desired that the switch 19 be maintainedin the pause mode P when there is no need to use the cordless telephoneset 10 and that the switch 19 be maintained in the standby mode P whenthe subscriber is to be notified of receipt of a call. When thesubscriber responds to a call, he or she switches the mode of the switch19 from the standby mode S to the active mode A. This switchingoperation is carried out if the subscriber calls another subscriber.

Generally, a battery charger is provided for the cordless telephone set.For example, a battery charger is built in the host device. FIG. 2 showsa battery charger 30 for use in the cordless telephone set 10 shown inFIG. 1A. The battery charger 30 has a power unit 31, a commercial powersupply plug 32, and a pair of terminals 33. The power unit 31 generatesa D.C. current from commercial power supplied via the plug 32. A pair ofthe terminals 33 is respectively connected to a pair of terminals 16 ofthe cordless telephone set 10 shown in FIG. 1 when the battery 15 ischarged. The D.C. current generated by the power unit 31 is supplied toa charging circuit 17 via the terminals 33 and 16. The charging circuit17 generates a battery charging current from the received D.C. currentand outputs it to the battery 15 via a diode 18. Energy consumed, forexample, in the standby mode S is compensated for by means of chargingby means of the charging circuit 17.

When the battery 15 is charged in the standby mode S, the subscriber canbe notified of receipt of a call. However, there is a possibility thatthe charging of the battery 15 is carried out in the pause mode P andnevertheless the subscriber wishes to be notified of receipt of a call.In this case, in conventional technology the subscriber is not notifiedof receipt of a call.

SUMMARY OF THE INVENTION

It is a general object of the present invention to provide a cordlesstelephone set in which the above disadvantage of the related cordlesstelephone set is eliminated.

A more specific object of the present invention is to provide a cordlesstelephone set capable of notifying the subscriber of receipt of a calleven if the built-in battery is being charged in the pause mode.

The above objects of the present invention are achieved by a cordlesstelephone set comprising: communications means for communicating withanother cordless telephone set a transmission medium including radiochannels, the communications means comprising a part that detects a callreceived via the transmission medium and indicates receipt of the call;a battery generating electricity necessary to operate the cordlesstelephone set; D.C. signal detection means for detecting a D.C. signalsupplied from a battery charger when the battery is coupled to thebattery charger and charged thereby; and power supply line settingmeans, coupled to the part of the communications means and the D.C.signal detection means, for establishing a power supply line forsupplying electricity generated by the battery charger to the part ofthe communications means when the D.C. signal detection means detectsthe D.C. signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description when readin conjunction with the accompanying drawings, in which:

FIG. 1A is a block diagram of a related cordless telephone set;

FIG. 1B is a block diagram of a host device coupled to the cordlesstelephone set via radio channels;

FIG. 2 is a block diagram of a battery charger for use in the cordlesstelephone set shown in FIG. 1A;

FIG. 3 is a block diagram showing an overview of a portable telephoneset according to a first embodiment of the present invention;

FIG. 4 is a block diagram illustrating the detail of the portabletelephone set shown in FIG. 3;

FIG. 5 is a circuit diagram of a current detection unit shown in FIG. 4;

FIG. 6 is a flowchart showing the operation of the portable telephoneset shown in FIG. 4;

FIG. 7 is a block diagram of a portable telephone set according to asecond embodiment of the present invention;

FIG. 8 is a block diagram showing an overview of a portable telephoneset according to a third embodiment of the present invention;

FIG. 9 is a block diagram illustrating the detail of the portabletelephone set shown in FIG. 8;

FIG. 10 is a circuit diagram of a voltage detection unit shown in FIG.9;

FIG. 11 is a block diagram of a portable telephone set according to afourth embodiment of the present invention; and

FIG. 12 is a circuit diagram of an electronic switch used in lieu of arelay used in the first through fourth embodiments of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 is a block diagram showing an overview of a cordless telephoneset according to a first embodiment of the present invention. In FIG. 3,parts that are the same as parts shown in the previously describedfigures are given the same reference numerals as previously. A cordlesstelephone set 1000 shown in FIG. 3 comprises a circuit block 99, acharging current detection block 100, and a power supply line settingblock 200. The circuit block 99 corresponds to a part of the structurewhich detects a call addressed to the portable telephone set 1000 andnotifies the subscriber of receipt of this call by means of a sounder orthe like. The charging current detection block 100 detects a D.C.current supplied from the battery charger 30 to the charging circuit 17.The power supply line setting block 200 establishes a power supply linefor supplying electricity to the circuit block 99 when the chargingcurrent detection block 100 detects the D.C current from the batterycharger 30. The power supply line established by the charge line settingblock 200 includes a line connected to the battery charger 30 andbypasses the charging circuit 17. Hence, even if the battery 15 is beingcharged in the pause mode, the circuit block 99 of the cordlesstelephone set 1000 can receive a call and indicate receipt of this call.

FIG. 4 is a block diagram showing the details of the cordless telephoneset 1000 shown in FIG. 3. In FIG. 4, parts that are the same as partsshown in the previously described figures are given the same referencenumerals as previously. The charging current detection block 100 shownin FIG. 3 comprises a current detection unit 23. The power supply linesetting block 200 shown in FIG. 3 comprises a logic OR gate 24 and arelay 25. The output terminal of the current detection unit 23 isconnected to the relay 25 and to the battery 15. The circuit block 99corresponds to the radio interface unit 1, the signal receiver 8 and theprocessor 11. One of the two input terminals of the OR gate 24 isdirectly connected to the battery 24, so that the OR gate 24 is alwaysactive. When the processor 11 is informed of receipt of a call by meansof the signal receiver 8, the processor 11 outputs a control signal tothe OR gate 24 via its control port. This control signal indicatesreceipt of the above call. An output terminal of the current detectionunit 23 is connected to the OR gate 24. An output terminal of the ORgate 24 is connected to a control terminal of the relay via a controlline 26.

When the battery 15 is charged, the terminals 33 of the battery charger30 are respectively connected to the terminals 16 of the portabletelephone set 1000, and the D.C. current generated by the power device31 is applied to the charging circuit 17 via the current detection unit23. The charging circuit 17 generates the charging current from thereceived D.C. current, and supplies it to the battery 15 via the diode18.

FIG. 5 is a circuit diagram of the current detection circuit 23 shown inFIG. 4. The D.C. current supplied from the power device 31 to thecharging circuit 17 passes through a light-emitting diode 232 of aphotocoupler 231. Hence, the light-emitting diode 232 turns ON atransistor 233 of the photocoupler 231, and the transistor 233 passes acurrent from a power supply line Vcc to the ground via a resistor 234.As a result, the potential of the transistor 233 is approximately equalto a ground potential. When the transistor 233 is ON, a base current isallowed to pass through a resistor 235 and the transistor 233. Hence,the transistor 236 is turned ON. In this state, a collector currentpasses through a resistor 237. Hence, the potential of the collector ofthe transistor 236 increases to approximately the voltage Vcc. Thecollector of the transistor 236 serves as the output terminal of thecurrent detection unit 23, which is connected to one of the two inputterminals of the OR gate 24. The collector potential of the transistor236 shown in FIG. 5 is applied to the control terminal of the relay 25via the OR gate 24 and the control line 26. When the collector potentialof the transistor 236 is almost equal to the ground level, the relay 25is OFF, so that the relay 25 is maintained in the open state. When thecollector potential of the transistor 236 is almost equal to the voltageVcc, the relay 25 is ON, so that the relay 25 is maintained in theclosed state. Further, as long as the processor 11 is outputting thecontrol signal to the OR gate 24, the relay 25 is maintained in the ONstate. As will be described later, the processor 11 continuously outputsthe control signal to the OR gate 24 until the cordless telephone set1000 is put offhook.

While the relay 25 is maintained in the conducting state, the outputterminal of the battery 15 is directly connected to the power supplyline 20 via the relay 25. Even if the switch 19 is in the pause state,electricity can be supplied from the battery 15 to the radio interfaceunit 1, the signal receiver 8 and the processor 11. In response to theringing tone generated by the sounder 14, the subscriber detaches theportable telephone set 1000 from the battery charger 30. Thereby, theterminals 16 of the portable telephone set 1000 are disconnected fromthe terminals 33 of the battery charger 30, and the D.C. current is nolonger supplied to the charging circuit 17. At this time, thetransistors 233 and 236 are turned OFF, and the ground potential isapplied from the current detection unit 23 to the OR gate 24. However,the processor 11 continues to supply the high-level control signal tothe OR gate 24, which is always ON by the supply of electricity from thebattery 15. Thus, the relay 25 is continuously maintained in the ONstate, and electricity is continuously supplied to the radio interfaceunit 1, the signal receiver 8 and the processor 11 via the relay 25.Then, the subscriber switches the mode from the pause mode P to theactive mode A. Thereby, electricity is supplied to all of the structuralelements of the cordless telephone set 1000, and the subscriber cancommunicate with the caller. Then, the subscriber will put offhook theportable telephone set. It is possible to design the switch 19 and thehook switch 13 so that the switch 19 automatically switches to theactive mode when the hook switch 13 is turned ON, and to the standbymode when the hook switch 13 is turned OFF.

FIG. 6 is a flowchart of the operation of the processor 11. The batterycharger 30 is connected to the portable telephone set 1000 set to thepause mode P (power ON; step S1). The processor 11 sets the level of itscontrol port connected to the OR gate 24 to a low (L) level (step S2).Then, the processor 11 waits for incoming or outgoing calls (step S3).When the processor 11 is informed by the signal receiver 8 of receipt ofa call (step S4), the processor 11 sets the control port connected tothe OR gate 24 to a high level (step S5). Then the processor 11 drivesthe sounder 14 so that the ringing tone is generated by the sounder 14.Then the processor 11 waits for a response (step S7). The subscriberchanges the mode of the switch 19 from the pause mode P to the activemode A (step S8), and puts the cordless telephone set 1000 offhook (stepS9). Alternatively, the subscriber puts the portable telephone set 1000offhook and turns the switch 19 from the pause mode P to the active modeA. Then, the processor 11 sets the control port connected to the OR gate24 to the low level (step S10). Then, speech communications start (stepS11). When the subscriber terminates the speech communications, the hookswitch 13 is turned OFF (step S12). Then, the subscriber changes themode from the active mode A to the pause mode P (step S13), and connectsthe portable telephone set 1000 to the battery charger 30.

When the subscriber calls another subscriber, he or she changes the modefrom the pause mode P to the active mode A (step S14) and puts theportable telephone set 1000 (step S15). Alternatively, at step S14, thesubscriber puts the portable telephone set 1000 offhook and then changesthe mode from the pause mode P to the active mode A second. Then, thesubscriber dials the telephone number of the called subscriber (stepS16), which is called by means of the sounder of the called subscriber(step S17). The processor 11 receives a response from the calledsubscriber (step S18), and executes step S10.

FIG. 7 is a block diagram of a second embodiment of the presentinvention. In FIG. 7, parts that are the same as parts shown in thepreviously described figures are given the same reference numerals aspreviously. The second embodiment of the present invention correspondsto a modification of the above-mentioned first embodiment thereof. Thesecond embodiment of the present invention is configured by adding twodiodes 27 and 28 to the configuration shown in FIG. 4. One of the outputterminals of the battery 15 is connected to the relay 25 via the diode27. One of the terminals 16 is connected to the relay 25 via the diode28. When the relay 25 is maintained in the conducting state, electricityfrom the battery charger 30 is directly supplied to the power supplyline 20 via the terminal 16 and the diode 28. Hence, the potential of anode, at which the diode 28 and the relay 25 are connected to each otherare connected to each other, is higher than that of the output terminalof the battery 15. In this state, the diode 27 is reversely biased andopencircuited. As a result, electricity from the battery 15 is notsupplied to the power supply line 20 at all. In this state, the battery15 is more efficiently charged.

A description will now be given of a third embodiment of the presentinvention. FIG. 8 is a block diagram showing an overview of a portabletelephone set 2000 according to the third embodiment of the presentinvention. In FIG. 8, parts that are the same as parts shown in thepreviously described figures are given the same reference numerals aspreviously. The third embodiment of the present invention uses acharging voltage detection block 300 in lieu of the charging currentdetection block 100 shown in FIG. 3.

FIG. 9 illustrates the details of the portable telephone set 2000 of thepresent invention shown in FIG. 8. In FIG. 9, parts which are the sameas parts shown in the previously described figures are given the samereference numerals as previously. A voltage detection unit 43 isconnected between the terminals 16. An output terminal of the voltagedetection unit 43 is connected to one of the two input terminals of theOR gate 24. The voltage detection unit 43 detects a D.C. voltagedeveloping between the terminals 16 connected to the terminals 33 of thebattery charger 30. The other parts of the configuration shown in FIG. 9are configured in the same manner as shown in FIG. 4.

FIG. 10 is a circuit diagram of the voltage detection unit 43 shown inFIG. 9. The voltage detection unit 43 shown in FIG. 10 comprises a fieldeffect transistor (hereinafter simply referred to as an FET) 44. A gateG and a drain D of the FET 44 are connected to each other and connectedto one of the two terminals 16. A source S of the FET 44 is connected toone of the two input terminals of the OR gate 24, and is connected tothe other terminal 16 through a bias resistor R. When the batterycharger 30 is connected to the portable telephone set 2000, the FET 44is turned ON, and one of the two input terminals of the OR gate 24 isset to a high potential obtained at the source S of the FET 44.

FIG. 11 is a block diagram of a portable telephone set 2000A accordingto a fourth embodiment of the present invention, which embodimentcorresponds to a modification of the third embodiment of the presentinvention shown in FIG. 10. The aforementioned diodes 27 and 28 areadded to the configuration shown in FIG. 10 in the same manner as shownin FIG. 7. The voltage detection unit 43 operates in the above-mentionedmanner, and the diodes 27 and 28 operate in the aforementioned manner.

The relay 25 comprises a movable terminal, a stationary terminal and acoil driving the movable terminal. However, it is possible to use anelectronic component in lieu of the above structure.

FIG. 12 is a circuit diagram of an electronic switch used in lieu of therelay 25. The electronic switch shown in FIG. 12 comprises a pnptransistor 51 and an npn transistor 52. One of the output terminals ofthe battery 15 is connected to an emitter of the transistor 51, and thecollector thereof is connected to the power supply line 20. The base ofthe transistor 51 is coupled to the collector of the transistor 52 via aresistor R1. The collector of the transistor 52 is also coupled to thepower supply line Vcc via a resistor R2. The emitter of the transistor52 is connected to, for example, the ground. The base of the transistoris connected to the output terminal of the OR gate 24 via the controlline 26.

The switch 19 can be formed with a mechanical component or an electroniccomponent.

The present invention is not limited to the specifically describedembodiments, and variations and modifications may be made withoutdeparting from the scope of the present invention. For example, thepresent invention includes a portable telephone set having a basicstructure other than the aforementioned basic structure. For example,the present invention includes a portable telephone set which isdirectly coupled to an exchange via radio channels, without the hostdevice.

What is claimed is:
 1. A cordless telephone setcomprising:communications means for communicating with another telephoneset via a transmission medium including radio channels, saidcommunications means comprising a part which detects a call received viathe transmission medium and indicates receipt of said call; a batterygenerating electricity necessary to operate the cordless telephone set;D.C. signal detection means for detecting a D.C. signal when the batteryis coupled to a battery charger and charged thereby; power supply linesetting means, coupled to said part of the communications means and saidD.C. signal detection means, for establishing a first power supply linefor supplying the electricity generated by at least one of the batteryand the battery charger to said part of the communications means whensaid D.C. signal detection means detects said D.C. signal; and switchmeans provided between the battery and the communications means forturning ON and OFF power supply from the battery to the communicationsmeans and establishing a second power supply line between the batteryand the communications means, wherein the first power supply lineestablished by said setting means is different from the second powersupply line established by said switch means to enable said call to bereceived even when said switch is OFF, and wherein the first powersupply line maintains supply of power from the battery to thecommunications means even if the cordless telephone set is detached froma battery charger when a call is received.
 2. A portable telephonesystem comprising:a portable telephone set; and a host device connectedto an exchange via a transmission line and connected to said portabletelephone set via radio channels, said portable telephone setcomprising: communications means for communicating with another cordlesstelephone set via the host device and the exchange, said communicationsmeans comprising a part which detects a call received via the hostdevice and indicates receipt of said call; a battery generatingelectricity necessary to operate the cordless telephone set; D.C. signaldetection means for detecting a D.C. signal when the battery is coupledto a battery charger and charged thereby; power supply line settingmeans, coupled to said part of the communications means and said D.C.signal detection means, for establishing a first power supply line forsupplying the electricity generated by at least one of the battery andthe battery charger to said part of the communications means when saidD.C. signal detection means detects said D.C. signal; and switch meansprovided between the battery and the communications means for turning ONand OFF power supply from the battery to the communications means andestablishing a second power supply line between the battery and thecommunication means, wherein the first power supply line established bysaid setting means is different from the second power supply lineestablished by said switch means to enable a call to be received evenwhen said switch means is OFF, and wherein the first power supply linemaintains supply of power from the battery to the communications meanseven if the cordless telephone set is detached from a battery chargerwhen a call is received.
 3. A cordless telephone setcomprising:communications means for communicating with another telephoneset via a transmission medium including radio channels, saidcommunications means comprising a part which detects a call received viathe transmission medium and indicates receipt of said call; a batterygenerating electricity necessary to operate the cordless telephone set;D.C. signal detection means for detecting a D.C. signal when the batteryis coupled to a battery charger and charged thereby; power supply linesetting means, coupled to said part of the communications means and saidD.C. signal detection means, for establishing a first power supply linefor supplying the electricity generated by at least one of the batteryand the battery charger to said part of the communications means whensaid D.C. signal detection means detects said D.C. signal; and switchmeans provided between the battery and the communications means forturning ON and OFF power supply from the battery to the communicationsmeans and establishing a second power supply line between the batteryand the communication means, wherein the communications means includesmeans for generating a signal which causes the power supply line settingmeans to maintain the first power supply line when the communicationsmeans detects a call to enable said call to be received even when saidswitch means is OFF; and wherein the first power supply line maintainssupply of power from the battery to the communications means even if thecordless telephone set is detached from a battery charger when a call isreceived.
 4. A cordless telephone set comprising:communications meansfor communicating with another telephone set via a transmission mediumincluding radio channels, said communications means comprising a partwhich detects a call received via the transmission medium and indicatesreceipt of said call; a battery generating electricity necessary tooperate the cordless telephone set; stopping means for stoppingsupplying of electricity from the battery to the communications means;D.C. signal detection means for detecting a D.C. signal when the batteryis coupled to a battery charger and charged thereby; electricity supplycontrol means for supplying electricity from the battery charger to thecommunications means when the D.C. signal detection means detects theD.C. signal even if said stopping means stops supplying of electricityfrom the battery to the communications means, wherein said electricitysupply control means comprises means for continuously supplying theelectricity to the communications means even if the communications meansdetects a call addressed to the cordless telephone set and the D.C.signal detection means cannot detect the D.C. signal.
 5. A cordlesstelephone set comprising:communications means for communicating withanother telephone set via a transmission medium including radiochannels, said communications means comprising a part which detects acall received via the transmission medium and indicates receipt of saidcall; a battery generating electricity necessary to operate the cordlesstelephone set; D.C. signal detection means for detecting a D.C. signalwhen the battery is coupled to a battery charger and charged thereby;power supply line setting means, coupled to said part of thecommunications means and said D.C. signal detection means, forestablishing a first power supply line for supplying the electricitygenerated by at least one of the battery and the battery charger to saidpart of the communications means when said D.C. signal detection meansdetects said D.C. signal; and switch means provided between the batteryand the communications means for turning ON and OFF power supply fromthe battery to the communications means and establishing a second powersupply line between the battery and the communication means, wherein thefirst power supply line established by said power supply line settingmeans is different from the second power supply line established by saidswitch means; and wherein: the first power supply line maintains supplyof power from the battery to the communications means even if thecordless telephone set is detached from a battery charger when a call isreceived; said switch means has a first mode, a second mode and a thirdmode; the electricity generated by said battery is supplied to saidcommunications means when said switch means is set in the first mode;the electricity generated by said battery is supplied to only said partof the communications means when said switch means is set in the secondmode; and the electricity generated by said battery is not supplied tosaid communications means at all when said switch means is set in thethird mode.
 6. A cordless telephone set comprising:communications meansfor communicating with another telephone set via a transmission mediumincluding radio channels, said communications means comprising a partwhich detects a call received via the transmission medium and indicatesreceipt of said call; a battery generating electricity necessary tooperate the cordless telephone set; D.C. signal detection means fordetecting a D.C. signal when the battery is coupled to a battery chargerand charged thereby; power supply line setting means, coupled to saidpart of the communications means and said D.C. signal detection means,for establishing a power supply line for supplying the electricitygenerated by at least one of the battery and the battery charger to saidpart of the communications means when said D.C. signal detection meansdetects said D.C. signal; and switch means provided between said batteryand said communications means and which has a first mode, a second modeand a third mode; wherein: the electricity generated by said battery issupplied to said communications means when said switch means is set inthe first mode; the electricity generated by said battery is supplied toonly said part of the communications means when said switch means is setin the second mode; the electricity generated by said battery is notsupplied to said communications means at all when said switch means isset in the third mode; and the power supply line maintains supply ofpower from the battery to the communications means even if the cordlesstelephone set is detached from a battery charger when a call isreceived.